The present invention relates to frequency coverage of a LC tank oscillator such as voltage controlled oscillator (VCO) or digitally controlled oscillator (DCO), and more particularly, to a tunable inductor.
When a mobile phone is designed to operate in multiple modes respectively corresponding to different telecommunication specifications, multiple DCOs may be required for being utilized in these modes, respectively. According to the related art, the DCOs may be integrated into a single DCO implemented by utilizing capacitance tuning, in order to save the chip area. However, some problems may occur. First of all, the tuning range of conventional DCO implemented by capacitive tuning relies on the parasitic capacitance of switch, which is limited by the maximum allowed signal swing for a specific process. Secondly, the conventional DCO implemented by capacitive tuning may suffer compromised performance whenever a large tuning range is required, since the majority of capacitance is allocated for tunable purpose which usually comes with more loss than non-tunable capacitance. Thus, there is a need for a tunable inductor, in order to implement a single DCO with large tuning range.